Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, in the development of such batteries, designs of new electrodes and batteries to improve capacity density and specific energy are mainly studied.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using a liquid electrolyte such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. For these reasons, the lithium secondary batteries have been advantageously used. In particular, since middle- or large-sized battery modules used for hybrid vehicles are desirably designed as small and light as possible, it is urgently demanded to design lithium secondary batteries with high power.
In addition, it is very important to ensure the safety of such electrochemical devices, particularly in consideration of influences or the like of the middle- or large-sized batteries to the human. The most important consideration is that the electrochemical device should not cause any damage to a user due to malfunction. In this aspect, when the electrochemical device is overheated to cause a run-away or the separator is damaged due to an external impact, the possibility of explosion is increased.
A polyolefin-based micro-porous film commonly used as a separator of an electrochemical device is extremely thermally shrunken at a temperature of 100° C. or above due to its material characteristics and the characteristics of its manufacturing process including elongation, which may cause a short between a cathode and an anode. Meanwhile, in order to prevent the insulation properties of the separator from being deteriorated due to leak current, porosity and pore size, and resultant air permeability are controlled to low levels, and thus ion conductivity is not so high, which becomes a factor of hindering realization of high-power batteries.
Thus, there is needed to develop a technique capable of increasing ion conductivity while improving thermal stability of a separator using a polyolefin-based micro-porous film and minimizing the generation of leak current.
Meanwhile, Korean laid-open patent publication Nos. 10-2006-0072065 and 10-2007-0000231 disclose a separator having a porous coating layer, which is made of a mixture of a binder polymer and filler particles such as inorganic particles and formed on at least one surface of a porous substrate having a plurality of pores such as a polyolefin-based micro-porous film. However, these documents just disclose the techniques relating to the improvement of thermal stability of a separator in accordance with the formation of a porous coating layer, but they do not teach the design of a porous substrate and correlations between the porous substrate and the porous coating layer, which may realize a high-power battery.